A spark plug is used to ignite an internal combustion engine such as a vehicle engine. Generally, the spark plug includes a tubular metal shell, a tubular insulator arranged in an inner hole of this metal shell, a center electrode arranged at the inner hole on a distal end side of the insulator, and a ground electrode whose one end is bonded to the distal end side of the metal shell and the other end includes a spark discharge gap with the center electrode. Further, the spark plug is spark-discharged at the spark discharge gap which is formed between the distal end portion of the center electrode and the distal end portion of the ground electrode, in a combustion chamber of the internal combustion engine to burn a fuel filled up in the combustion chamber.
As a material forming the center electrode and the ground electrode, a Ni alloy or the like is generally used. Regarding oxidation resistance and wear resistance, the Ni alloy is slightly inferior to a precious metal alloy whose main constituent is a precious metal such as Pt and Ir. However, because of its inexpensiveness compared with the precious metal, the Ni alloy is preferably used as the material forming the ground electrode and the center electrode.
Recently, there has been a trend of high-temperature of a temperature in the combustion chamber. Therefore, if spark discharge occurs between the distal end portion of the ground electrode and the distal end portion of the center electrode made of the Ni alloy or the like, each distal end portion of the ground electrode and the center electrode opposed to one another is likely to generate spark erosion. Therefore, there has been developed a method for improving the wear resistances of the ground electrode and the center electrode by disposing tips at each distal end portion of the ground electrode and the center electrode opposed to one another to generate the spark discharge at the tips.
As the material forming the tips, a material whose main constituent is a precious metal excellent in the oxidation resistance and spark erosion resistance is often used. The material includes Ir, an Ir alloy, a Pt alloy, or the like.
For example, Japanese Patent No. 3672718 discloses a spark plug that uses an IR—Rh alloy as a material of a firing end. Specifically, Japanese Patent No. 3672718 discloses the spark plug that includes a precious metal tip “formed from an alloy containing Ir as a main component, Rh in an amount of 0.1 wt. % to 35 wt. %, and at least one of Ru and Re in an amount of 0.1 wt. % to 17 wt. % in total.” Objects of this invention are the following two points. An object is to provide a spark plug that shows remarkably less susceptibility to wear of a firing end stemming from oxidation/volatilization of Ir constituent at high temperatures as compared with a conventional Ir—Rh alloy, and can secure excellent durability in traveling in an urban area as well as in high speed driving. The other object is to provide a spark plug that can contain a smaller amount of expensive Rh than that of a conventional one and secure durability with low costs (claim 1 and paragraph 0006 in Japanese Patent No. 3672718).
Japanese Patent No. 4672551 discloses a spark plug that includes the precious metal tip “containing Ir as a main component, 0.5 to 40 mass % of Rh, and 0.5 to 1 mass % of Ni, and further containing at least one of Pt and Pd by 4 to 8 mass %” to provide the spark plug that can suppress sweating and peeling of precious metal in a surface of the discharge portion while suppressing spark erosion, oxidative consumption, and abnormal erosion of the discharge portion (claim 1 and paragraph 0006 in Japanese Patent No. 4672551).
Recently, a spark plug that can support various driving styles has been required. That is, a spark plug having excellent durability under any conditions, such as a condition putting emphasis on an output under low oxygen concentration atmosphere by increasing a mixing ratio of fuel to air and a condition putting emphasis on fuel economy under high oxygen concentration atmosphere by decreasing the mixing ratio of the fuel to the air, has been required.
An evaluation of the conventional tip with such viewpoint found the following problem. The inventors examined a composition of the tip that can reduce the oxidative consumption and found the following. The tip made of an Ir—Rh—Ru alloy containing Ir as a main component, Rh, and Ru was able to reduce the oxidative consumption at an Air/Fuel ratio of around 12 and an inside of a combustion chamber being under the low oxygen concentration atmosphere. However, at the Air/Fuel ratio of around 14 and the inside of the combustion chamber being under the high oxygen concentration atmosphere, which have been conventionally put emphasis on, the oxidative consumption proceeded and sufficient durability was not able to be obtained.
An advantage of this invention is a spark plug including a tip provided on at least one of the center electrode and the ground electrode and featuring good durability by reducing oxidative consumption without an influence from oxygen concentration under an environment of this tip being exposed.